Files piling up in /var/* directories

[Charles]

I've noticed that I have files piling up in /var/cache, /var/log, and /var/lib.  These three folders now have 100 mb of files in them. Which of these must be present for the machine to run?  This is (at the moment) a home machine; I don't *need* the  log files, but I don't know what happens if I just delete the files in these three directories.